Embodiments of the invention relate generally to alternating current (AC) motors and, more particularly, to a system and method for estimating the speed of an AC motor without an encoder.
Motor soft starters are devices that control application of voltage or current from an AC power source to an induction motor. Soft starters are configured to limit the transient current inrush to the induction motor during start-up, resulting in a “soft” motor starting without affecting the power quality of grid. In operation, power from the AC source is passed through switching devices in the soft starter, such as a pair of anti-parallel or back-to-back solid-state switches in the form of thyristors or silicon controlled rectifiers (SCRs), to control the current flow and, in turn, the terminal voltages of the induction motor.
In general, the soft starter temporarily reduces current in the motor during startup via selective control of the thyristors without this control the motor currents can reach 6 to 8 time rated current. This reduction allows for reduced stresses on the motor and electrical network, which increases the life of the system. The thyristors are “triggered” or “fired” (controlled to turn-on) at a given angle, γ, as measured when the voltage becomes positive. The resulting currents flow through the given phases until they reach zero, at which point the thyristors turn off. This pattern creates a “notch” in the voltage. The larger the notch width, the smaller the rms voltage applied to the motor. Since torque is a function of the square of the rms voltage, the larger the notch width, the smaller the torque. If the notch width is zero, full voltage is applied to the motor. Typically, the soft starting process lasts a few seconds, at the end of which the system reaches its final speed and contactors are closed to bypass the SCRs.
Knowledge of the rotational speed of the AC motor is useful for optimizing motor starting controls and operation of AC motors both during the motor start up process and thereafter. Existing motor speed estimation methods for AC motors often depend on high-frequency current injection, which is not feasible with an AC motor driven by a thyristor-based soft starter. Other known methods rely on complicated observers, motor equivalent models, or both, which are largely impractical on thyristor-based soft starter systems, especially soft starters that operate only for a short period of time. For example, one known technique that estimates motor speed relies on a Fast Fourier Transform (FFT) analysis of the current waveforms and slight deviations in the current waveforms due to machine rotor winding asymmetries is computationally intensive and lacks in generality because motor asymmetries vary from one machine to another. Another known method of estimating speed uses a model-reference adaptive system that is torque-based for a first portion of the range from start to nominal speed and flux-based for the remaining portion of the speed range. This model-based technique is computationally intensive and depends on knowledge of numerous machine parameters, resistances, and inductances. Yet another known technique skips a triggering event in one phase to induce transients in the current and phase lag. However, the small magnitude of the resulting signal is difficult to monitor.
While some of the aforementioned techniques skip SCR triggering or firing events in order to estimate motor speed, these techniques do not make any provision for the dangerous current transients that may be induced when resuming SCR firing after some interruption on a machine that is rotating. After disconnection, the speed of the motor costs down and the back-emf generated is no longer at the same frequency or phase as the mains voltage. If the mains voltage and the back-emf are out of phase when a live reclosing is attempted, a voltage of up to two times the mains voltage will be applied to the AC machine, resulting in high and potentially destructive transient currents.
Accordingly, it would be desirable to have a sensorless system capable of estimating the rotational speed of a thyristor-driven AC motor that does not rely on knowledge of operating machine parameters for a particular AC motor or operate using a computationally intensive technique. It would also be desirable for such a soft starter system to acquire rotational speed estimates without inducing potentially destructive transients in the system.